Three-Phase Inverter for Grid Current SPV System

This MPPT DC-DC converter is followed by a DC-AC inverter for grid con­nection or to supply power to the AC loads in stand-alone applications. The basic operation principle of the DC-AC inverter is to keep the dc-link voltage at a reference value meanwhile keep the frequency and phase of output current are same as grid voltage. The error signal generated from voltage comparison is adjusted by voltage adjuster and it decides the value of reference current, then it’s used to switch ON and OFF the values of the inverter. The load of the grid – connected inverter is power grid, grid power is controlled by grid current [43]. Figure 5.12 shows the schematic of three-phase gird connected inverter. Assume

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Fig. 5.12 Schematic of three-phase grid connected inverter

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Fig. 5.13 Generalized control structure of the SPV system

that three-phase grid voltage is symmetrical, stable and internal resistance is zero; three phase loop resistance RS and LS are of the same value respectively; switching loss and on-state voltage is neglectable; affection of distribution parameter is neglectable; switching frequency of the rectifier is high enough.

The generalized control structure of SPV system is shown in Fig. 5.13. Following are the three different classes of control functions of power elec­tronics converter of SPV system.

1. Basic functions-common for all grid connected inverters • Grid current control

– THD limits imposed by standards

– Stability in the case of large grid impedance variations

– Ride through grid voltage disturbances

• DC voltage control

– Adaptation to grid voltage variation

– Ride-through grid voltage disturbance

• Grid synchronization

– Operation at the unity power factor as required by standards

– Ride through grid voltage disturbances

2. PV specific functions-common for all PV inverters

• Maximum power point tracking (MPPT)

– Very fast MPPT efficiency during steady state (typically >99 %)

– Stable operation at very low irradiance levels

• Anti-islanding (AI), as required by standards (VDE 0126, IEEE 1574, etc.)

• Grid monitoring

– Synchronization

– Fast voltage/frequency detection for passive AI

• Plant monitoring

– Diagnostic of PV panel array

– Partial shading detection

3. Ancillary functions

• Grid support

– Local voltage control

– Q compensation

– Harmonic compensation

– Fault ride through.

Updated: October 23, 2015 — 12:41 pm